Etching of substrates using a plasma is known. It is used principally with substrates made of silicon for semiconductor technology, in particular in the manufacture of chips from silicon wafers, in which patterns defined by etching masks, for example valleys, are etched into the substrate. The etching masks are masking layers, for example photoresist layers, applied onto the surface of the substrates. The plasma is ignited by exciting reactive gases or gas mixtures using high-frequency electromagnetic radiation. This is done, for example, by using an inductively coupled plasma source (ICP source) with high-frequency excitation. A typical ICP source has an excitation coil, placed around the plasma volume and having one or more turns, through which a high-frequency current, for example at a frequency of 13.56 MHZ, flows in order to excite the plasma. One end of the coil (the "hot" end) is thus connected to a high-frequency source.
An etching method of the species for deep silicon etching using an ICP source is described in German Patent Application No. 42 41 045.
The alternating field or magnetic field of the excitation coil is, however, inhomogeneous. This inhomogeneity causes an increase in the quantity of ions and reactive particles at the edge of the plasma as compared with the middle. Because of this excitation geometry, different etching rates are observed in the center and at the edge of the substrate. For example, the etching rate in the middle of a wafer with a diameter of 150 mm can be up to 20% less than in the edge region of the substrate.
In addition, the electrical leakage fields proceeding from the "hot" end of the coil that is at high voltage lead to a correspondingly severe deformation of the inductive plasma. At the same time, the plasma is displaced out of the center of the excitation coil toward the "hot" end. Ablation of the etching mask is accordingly also highly nonuniform, and is displaced from the center of the substrate toward the edge region. Ablation is less in the region of the substrate that is adjacent to the end of the excitation coil in the plasma source that is at high voltage than in the regions of the substrate remote from the "hot" coil end. These inhomogeneities result in inaccuracies in the etched structures.